A plasma treatment device is a device that generates a plasma discharge by supplying RF power to the electrodes, arranged in a reactor in which plasma is generated, and performs surface treatment for a substrate using the generated plasma. The plasma treatment device is used for semiconductor fabrication and so on.
FIGS. 11 and 12 are diagrams showing an example of the configuration of a plasma treatment device. FIG. 11 shows a simplified equivalent circuit. In the example of the configuration shown in FIG. 11, a plasma treatment device 100, configured by connecting an RF generator 101 to the electrodes of a reactor 102 via a feed cable 104 and an impedance matching box (matching box) 103, applies an ignition voltage to the load end (input end of the load side) to generate plasma and, after generating plasma, supplies RF power to the plasma load.
The RF generator 101 can be configured equivalently by an RF generator voltage 101a (Vg), an internal impedance 101b (Zg), and a low-pass filter 101c (LPF).
The matching box 103 achieves impedance matching between the RF generator 101 and the load side to reduce a reflected wave power transmitted from the load side to the RF generator. By doing so, the matching box 103 increases the supply efficiency of the predetermined-frequency RF power supplied from the RF generator 101 to the load.
FIG. 12 is a diagram showing an example of the configuration in which a plurality of RF generator voltages 101a (Vg) are used. The figure shows the configuration of a general RF generator 111 in which the output voltages of the plurality of RF generator voltages 101a are combined by a combiner 112 and are output via a low-pass filter 113. In the circuit configuration shown in FIG. 12, the RF generators, each composed of the RF generator voltage 101a (Vg), internal impedance 101b (Zg), and low-pass filter 101c (LPF), are connected to the feed cable 104 and the matching box 103 via the combiner 112 and the low-pass filter 113.
When power is supplied to a plasma load, it is known that the discharge voltage is decreased at the load end when the plasma discharge is started. Therefore, when a sufficient ignition voltage is not applied, a decrease in the load-end discharge voltage sometimes results in an unstable plasma ignition. To make plasma ignition stable, the RF generator device, which supplies power to the plasma load, is required to apply a voltage, high enough as an ignition voltage (firing voltage) for generating plasma, to the load end.
Conventionally, a class C RF generator is primarily used as an RF generator because, as compared to a class A RF generator or a class B RF generator, a class C RF generator is efficient and can amplify power to high power with a simple circuit.
In general, when electric power is supplied from an RF generator device to a load via a feed cable to supply power, the following two are known. One is that the load-end voltage depends on the electrical length that is determined by the cable length of the feed cable. The other is that the load-end voltage can be increased by determining the electrical length of the feed cable so that a predetermined relation is satisfied according to the wavelength of the RF power.
The following literatures (Patent Literatures 1-3) describe technologies for performing impedance adjustment and higher-harmonic component matching based on the electrical length LE determined by the cable length Lw of the feed cable 104 and the wavelength of the RF power.
Patent Literature 1 proposes a plasma CVD device that uses a variable-length coaxial cable as an impedance adjuster to supply power by reducing reflected wave power against the RF generator.
Another known problem is that a standing wave is generated from a forward wave and a reflected wave, which are higher-harmonic components mixed on the transmission line, due to a mismatch among the higher-harmonic components in the RF power. This standing wave indeterminately varies the generation and the distribution characteristics of plasma and, as a result, decreases process reproducibility and reliability in plasma. To address this problem, the following plasma treatment devices are proposed. In one plasma treatment device, the line length of the transmission line made is shorter than λ/2 or 3λ/4 of the wavelength λ of the third higher harmonics of the RF power (See Patent Literature 2). In another plasma treatment device, the cable length is predefined according to the process condition so that the higher-harmonic component level of the reflected wave power becomes equal to or lower than level of the fundamental wave (see Patent Literature 3).